Stimulation of 1,2-diacylglycerol accumulation in hepatocytes by vasopressin, epinephrine, and angiotensin II

Effects of EGF on the mass of inositol 1,4,5-trisphosphate and SN(1,2)-diacylglycerol in freshly isolated rat hepatocytes: comparison with vasopressin

We measured the masses of inositol 1,4,5 trisphosphate (Ins(1,4,5)P3) and diacylglycerol (DAG) in hepatocytes in response to both epidermal growth factor (EGF) and vasopressin. EGF at 25 nM did not alter Ins(1,4,5)P3 content of hepatocytes. However, the combination of 100 nM EGF concentration and incubation with lithium did increase Ins(1,4,5)P3 content. This increase was only one tenth of that elicited by vasopressin in parallel incubations. This finding resolves a controversy concerning the ability of EGF to increase Ins(1,4,5)P3 in hepatocytes, and argues against a role for phosphoinositide hydrolysis in EGF action in hepatocytes. Both EGF and vasopressin caused a rapid (30 s) increase in DAG content. A delayed increase in DAG content, that was maximal after several minutes, was observed only for vasopressin. The rapid increase in DAG content implies an activation of protein kinase C for both EGF and vasopressin.

CDC25-dependent induction of inositol 1,4,5-trisphosphate and diacylglycerol in Saccharomyces cerevisiae by nitrogen

The addition of ammonium sulfate to starved yeast cells leads to a 3- to 4-fold rapid increase of the second messengers inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG), the products of phosphoinositide-specific phospholipase C (PI-PLC). This response is reduced by dissecting the RAS-activating Cdc25 protein, and is completely abolished by the cdc25-1 mutation even at permissive temperature. Starved cdc25-1 mutant cells have a strongly reduced IP3 content, but an at least 10-fold increased DAG level compared to the isogenic wild-type strain. NH4 does not stimulate cAMP synthesis, and glucose does not induce IP3 and DAG. Our data suggest that the Cdc25 protein controls a nitrogen-specific signalling pathway involving the effector PI-PLC, in addition to the glucose-induced activation of adenylyl cyclase (AC).

Ca2+ ionophore and phorbol ester stimulate diacylglycerol formation and phosphatidylcholine hydrolysis in rat parotid acinar cells

We investigated the effects of A23187 and phorbol 12,13-dibutyrate (PDBu) on sn-1,2-diacylglycerol (DAG) accumulation and phosphatidylcholine (PC) hydrolysis in rat parotid acinar cells. Both A23187 and PDBu, in concentration ranges of 0.001-0.1 microM, stimulated DAG accumulation and PC hydrolysis in a time- and concentration-dependent manner. Treatment with A23187 and PDBu stimulated the release of [3H]choline and [3H]phosphocholine into the medium, indicating [3H]PC hydrolysis is due to the activation of phospholipases C and D; however, [3H]phosphatidylethanolamine hydrolysis was not indicated. These releases were unaffected by the addition of glucose 6-phosphate, a phosphatase inhibitor. Staurosporine, a protein kinase C inhibitor, significantly inhibited the DAG accumulation and the PC hydrolysis stimulated by these agents. Combinations of A23187 and PDBu potentiated the stimulatory effect which each of these agents alone had on DAG accumulation and PC hydrolysis. This mode of action was additive but not synergistic. These results suggest that DAG accumulation induced by A23187 and PDBu is related to the PC hydrolysis mediated via the activation of phospholipases C and D, and that it is not related to phosphatidylethanolamine hydrolysis.

Mechanism of carbachol-stimulated diacylglycerol formation in rat parotid acinar cells

We studied the relationship between phosphoinositide hydrolysis, phosphatidylcholine hydrolysis, and sn-1,2-diacylglycerol (DAG) formation in response to carbachol stimulation in rat parotid acinar cells. Previously, we demonstrated that DAG formation stimulated with 1 microM carbachol was biphasic: the first peak occurred at 5 min and the second one at 20 min. It was also demonstrated that the second peak was regulated in part by a calmodulin/protein kinase C-dependent mechanism. Based on the kinetic analysis of DAG formation and [32P]phosphoinositide breakdown, the first peak of carbachol (1 microM)-stimulated DAG accumulation was found to be related to the breakdown of [32P]phosphatidylinositol 4-monophosphate ([32P]PIP) and [32P]phosphatidylinositol 4,5-bisphosphate ([32P]PIP2). The second peak was found to be related to [32P]PIP2 breakdown. Carbachol stimulated the release of [3H]phosphocholine into the medium, indicating that the predominant pathway for phosphatidylcholine hydrolysis was via phospholipase C. Moreover, carbachol stimulated the release of [3H]choline metabolites in a time- and dose-dependent manner. This agonist slightly stimulated the release of [3H]ethanolamine metabolites. A calmodulin/protein kinase C-dependent mechanism was also studied and was found to be involved in carbachol-stimulated phosphatidylcholine hydrolysis; W-7, a calmodulin inhibitor and staurosporine, a protein kinase C inhibitor, inhibited the carbachol (1-microM)-induced release of [3H]choline metabolites at 20 min in a dose-dependent manner, but did not have inhibitory effects at 5 min. These results suggest that the first peak of DAG accumulation induced by carbachol is predominantly associated with the breakdown [32P]PIP and of [32P]PIP2 and that the second peak is predominantly associated with [32P]PIP2 breakdown and phosphatidylcholine hydrolysis.

1,2-Diacylglycerol and ceramide levels in rat liver and skeletal muscle in vivo

1,2-Diacylglycerol and ceramide levels were measured in liver and skeletal muscle of rats under the following four experimental conditions: 1) during rapid fetal growth in the second half of gestation and during postnatal aging, 2) during tissue anoxia lasting up to 10 min, 3) during fasting for up to 6 days, and 4) during stress induced by injection of endotoxin (lipopolysaccharide from Salmonella enteritidis). 1,2-Diacylglycerol and ceramide levels in fetal liver were 77 and 58% lower, respectively, than those of young rats after weaning. 1,2-Diacylglycerol and ceramide concentrations in liver of postnatal rats were not influenced by aging. Anoxia produced a 41-64% increase in liver 1,2-diacylglycerol levels and a 9-21% increase in liver ceramides. Fasting of adult rats for up to 6 days had no effect on 1,2-diacylglycerol and ceramide levels in liver. However, fasting increased skeletal muscle concentrations of 1,2-diacylglycerol and ceramides 88 and 44%, respectively. Injection of endotoxin had a biphasic effect on liver 1,2-diacylglycerol levels, causing a transient 52% increase at 1 h, followed by a 24-38% decrease below the control level at 17 h after endotoxin administration. Liver ceramide levels were increased 66, 52, and 27% at 1, 3, and 5 h after endotoxin, respectively, but did not differ from control at 17 h after injection. Endotoxin had no effect on muscle 1,2-diacylglycerol and ceramide concentrations at any interval.

Vasopressin stimulates pyruvate utilization through a Ca(2+)-dependent mechanism and lactate formation by a protein kinase C-dependent mechanism in isolated rat hepatocytes

Vasopressin stimulates lactate production by hepatocytes from fed rats, an effect which has been attributed exclusively to Ca2+ activation of glycogenolysis. We provide evidence here for two further actions of vasopressin which affect lactate formation by rat hepatocytes. In the presence of 50 mM glucose, vasopressin inhibited lactate production by hepatocytes. The inhibition was relieved by the presence of alpha-cyano-4-hydroxycinnamate (alpha-CHC), which blocks mitochondrial pyruvate transport. This suggests that vasopressin stimulates pyruvate utilization in the presence of a high concentration of glucose. Epidermal growth factor (EGF), which also increases lactate formation by hepatocytes, did not similarly decrease lactate accumulation in the presence of high glucose, suggesting no stimulation of lactate and pyruvate utilization by this hormone. In cells depleted of Ca2+, vasopressin also stimulated lactate formation. Although vasopressin did not cause the apparent translocation of protein kinase C between cell spaces, phospholipase C treatment of hepatocytes did duplicate vasopressin stimulation of lactate formation, provided fatty acid oxidation was suppressed by the simultaneous presence of the inhibitor palmixorate. We conclude that three actions of vasopressin affect lactate and pyruvate formation: the calcium-linked activations of glycogenolysis and mitochondrial pyruvate utilization, and a stimulation of glycolysis likely mediated by protein kinase C.

Angiotensin II and atrial natriuretic factor receptor interactions at the blood-brain barrier

Data from several laboratories indicate that cerebral endothelial cells possess cell surface receptors for numerous vasoactive agents including angiotensin II (AII) and atrial natriuretic factor (ANF). The intracellular messengers of these receptors as well as possible receptor interactions were explored. ANF increased cGMP 10-fold over basal levels while incubation of the microvessels with AII did not significantly affect the level of this nucleotide. In contrast, AII significantly potentiated the increase in cGMP by ANF. Incubation of cerebral microvessels with AII resulted in a significant increase in the intracellular mediator of PI hydrolysis, 1,2-diacylglycerol (DG). ANF had no affect on DG or on the AII mediated increase of DG. Finally, data at the level of receptor binding indicated that while ANF decreased [3H]angiotensin binding to cerebral microvessels, AII had no effect on the binding of ANF to its receptor. The results of the present study demonstrate that AII can potentiate the regulation of cGMP by ANF and suggest the possibility of receptor interactions in control of blood-brain barrier function.

Membrane-associated phospholipase D activity in rat sciatic nerve

Rat sciatic nerve contains a membrane-bound phospholipase D that catalyzes the hydrolysis of exogenous phosphatidylcholine (PC) to phosphatidic acid (PA) and choline. The enzyme is associated with a particulate fraction consisting primarily of microsomes and myelin. This fraction also contains phosphatidate phosphohydrolase activity leading to the production of diacylglycerols (DAG). The phosphohydrolase activity can be completely inhibited by NaF. Hydrolysis of exogenous PC requires detergent and is linear up to about 40 micrograms of protein at a pH optimum of 6.5. In the absence of NaF, the sum of PA and DAG increases linearly for 40 min, whereas in its presence, PA production is linear for only 15 min. At optimum conditions, PC hydrolysis proceeds at 15 nmol/h/mg of protein. Addition of increasing amounts of ethanol to the incubation system leads to the generation of increasing amounts of phosphatidylethanol, indicating transphosphatidylation activity. At an ethanol concentration of 0.4 M, phosphatidylethanol represents about one-half of the reaction products generated at approximately the same rate of enzymic activity observed in the absence of ethanol. Higher ethanol concentrations are inhibitory.

Evidence that the rate of phosphatidylcholine catabolism is regulated in cultured rat hepatocytes

The regulation of phosphatidylcholine (PC) catabolism has been studied in choline-deficient rat hepatocytes. Supplementation of choline-deficient hepatocytes, prelabeled with [3H]choline, with 100 microM choline increased the rate of PC catabolism by approx. 2-fold. The major product of PC degradation was glycerophosphocholine in both choline-deficient and choline-supplemented cells. Choline supplementation decreased the radioactivity recovered in lysoPC by 50%. This effect was accompanied by a 2-fold increase of labeled glycerophosphocholine. Comparable results were obtained when PC of the cells was prelabeled with [3H]methionine or [3H]glycerol. The activity of phospholipase A in cytosol, mitochondria and microsomes isolated from choline-deficient rat liver was similar to the activity in control liver, when determined with [3H]PC vesicles as the substrate. Measurement of the activity of phospholipase A with endogenously [3H]choline-labeled PC showed that the formation of lysoPC in mitochondria isolated form choline-supplemented cells was 40% lower than in choline-deficient cells. Alternatively, the formation of [3H]glycerophosphocholine and [3H]choline in microsomes from choline-supplemented cells was significantly higher (1.4-fold) than in microsomes from choline-deficient cells. These results suggest that the rate of PC catabolism is regulated in rat hepatocytes and that the concentration of PC might be an important regulatory factor.

Treatment of intact hepatocytes with synthetic diacyl glycerols mimics the ability of glucagon to cause the desensitization of adenylate cyclase

Incubation of intact hepatocytes with either of the synthetic diacyl glycerols 1-oleoyl-2-acetyl glycerol (OAG) or dihexanoyl glycerol (DHG) caused the transient uncoupling of the ability of glucagon to stimulate adenylate cyclase in membranes prepared from those cells. No change occurred in either the activity of the catalytic unit of adenylate cyclase or the coupling of Gs to adenylate cyclase. Diacyl glycerol action appeared to mimic glucagon-mediated desensitization of adenylate cyclase, suggesting that protein kinase C activation may provide the molecular trigger for glucagon desensitization.

Sources of diradylglycerols generated during cell growth and phorbol ester stimulation in Madin-Darby canine kidney cells

The molecular species of diacylglycerol and alkylacylglycerol of Madin-Darby canine Kidney (MDCK) cells were analyzed to determine the sources of diradylglycerols generated during cell growth and phorbol ester stimulation. MDCK cells in log phase growth contained higher levels of diacylglycerol and alkylacylglycerol than confluent cells. Both subclasses of diradylglycerol showed higher levels of saturated and monoenoic species during log phase. Glycerol incorporation into diradylglycerols was increased during growth, consistent with an increase in their synthesis de novo. Stimulation with 12-O-tetradecanoylphorbol-13-acetate (TPA), a potent activator of protein kinase C, caused an increase in the level of diacylglycerol but not alkylacylglycerol. Log phase MDCK cells showed a greater response to TPA treatment than confluent cells. The molecular species of diacylglycerol generated during stimulation with either TPA or dioctanoylglycerol closely resembled the species of phosphatidylcholine. These results indicate that TPA and synthetic diacylglycerol stimulate endogenous diacylglycerol production through the hydrolysis of phosphatidylcholine. In contrast, the higher content of diacylglycerol and alkylacylglycerol in replicating MDCK cells is the result of an increase in their synthesis de novo.

Protein kinase C-dependent diacylglycerol formation is mediated via Ca2+/calmodulin in parotid cells

The kinetics of carbachol-induced sn-1,2-diacylglycerol (DAG) formation and the underlying mechanism(s) involved in parotid acinar cells were investigated. Supramaximal concentrations of carbachol for amylase secretion (10 microM) caused a transient rise in DAG levels at 10 s. In contrast, this rapid rise was not elicited by 1 microM carbachol, which is the maximally effective concentration for amylase secretion. Carbachol (10 microM) also increased DAG levels linearly up to 20 min, which were sustained for up to a further 10 min. DAG formation stimulated by 1 microM carbachol was biphasic; the first peak was observed after 5 min and the second after 20 min. DAG formation induced by 0.01-0.1 microM carbachol was concentration-dependent and monophasic, peaking at 5 min. The second peak evoked by carbachol was partly inhibited by Ca2+ deprivation from the extracellular space, whereas the first peak was not. Similar results were obtained in experiments using Ca2+ antagonists such as verapamil and LaCl3. The protein kinase C inhibitors, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and staurosporine, and a calmodulin antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide (W-7), significantly inhibited the second DAG peak produced by 1 microM carbachol, but did not alter the first peak. The degree of inhibition of the second peak by these antagonists was comparable. Furthermore, the inhibitory effect of staurosporine and W-7 was concentration-dependent. The A23187-induced accumulation of DAG also was abolished by both staurosporine and W-7. These data indicate that a protein kinase C-dependent mechanism(s) is involved in mediating the second DAG accumulation peak induced by 1 microM carbachol and is mainly regulated by the Ca(2+)-calmodulin complex.

Effect of ethanol on phospholipid turnover and calcium mobilization in chick embryos

Effects of ethanol on [3H]inositol and [14C]choline incorporation into phosphatidylinositol (PI) and phosphatidylcholine (PC), free intrasynaptosomal Ca2+ ([Ca2+]i) and synaptosomal 45Ca2+ uptake, were investigated in the brain and heart of 17-day-old chick embryos to which a 10% ethanol solution had been injected on the 3rd day of embryogenesis. In brain synaptosomes, ethanol increased the incorporation of [3H]inositol and [14C]choline into PI and PC, increased [Ca2+]i, and decreased 45Ca2+ uptake. On the other hand, in heart synaptosomal membrane, ethanol decreased the incorporation of [3H]inositol and [14C]choline into PI and PC, decreased [Ca2+]i, and increased 45Ca2+ uptake. Ethanol stimulated in vitro [3H]inositol and [14C]choline incorporation into PI and PC in the brain and heart in both the control and ethanol-treated groups. However, addition of ethanol did not affect the release of 45Ca2+ from the synaptosomal membrane of either organ in either group. Addition of ethanol inhibited 45Ca2+ uptake in a dose-dependent manner in the brain but not in the heart. In both organs, there was a relationship between phospholipid turnover and [Ca2+]i after ethanol.

Sequential degradation of choline phosphoglycerides by phospholipase D and phosphatidate phosphohydrolase in dibutyryl cAMP-differentiated U937 cells

Dibutyryl-cAMP-differentiated U937 cells incorporate alkyllyso-sn-glycero-3-[32P]phosphocholine (alkyllyso-[32P]GPC) into cellular alkylacyl-sn-glycero-3-phosphocholine (alkylacyl-GPC). Upon stimulation with fMet-Leu-Phe (fMLP), recombinant C5a, or phorbol 12-myristate 13-acetate (PMA), these cells produce alkylacyl-sn-glycero-3-[32P]phosphate (alkylacyl-[32P]GP). In the presence of ethanol (0.5%), alkylacyl-sn-glycero-3-[32P]phosphoethanol (alkylacyl-[32P]GPet) is also formed with a concomitant reduction in alkylacyl-[32P]GP accumulation. Because cellular ATP is not labeled with 32P, alkylacyl-[32P]GP and alkylacyl-[32P]GPet must be formed by phospholipase D (PLD)-catalyzed hydrolysis and transphosphatidylation, respectively. Activation by receptor agonists, but not by PMA, requires extracellular Ca2+ and is augmented by cytochalasin B pretreatment. Upon stimulation, dibutryl cAMP-differentiated U937 cells labeled with alkylacyl-[32P]GPC produce [32P]PO4 but not [32P]phosphocholine. Furthermore, when these cells were labeled in alkylacyl-GPC by incubation with [3H]alkyllyso-GPC and then stimulated, [3H]alkylacyl-glycerol ([3H]alkylacyl-Gro) is produced with a time-course similar to that of [32P]PO4 formation and coincident with the decline in alkylacyl-GP accumulation. These results demonstrate that alkylacyl-GP formed by PLD is dephosphorylated by phosphatidate phosphohydrolase to produce PO4 and alkylacyl-Gro. Upon stimulation with fMLP or C5a, U937 cells labeled in diacyl-sn-glycero-3-phosphocholine (diacyl-GPC) by incubation with [3H]acyllyso-GPC generate [3H]diacyl-GP, [3H]diacyl-GPEt, and [3H]diacyl-Gro with kinetics similar to those for the generation of the [3H]alkyl products. Thus, in differentiated U937 cells stimulated with receptor agonists, both alkylacyl-GPC and diacyl-GPC are sequentially metabolized by PLD and phosphatidate phosphohydrolase.

Colony stimulating factor-1 stimulates diacylglycerol generation in murine bone marrow-derived macrophages, but not in resident peritoneal macrophages

Colony stimulating factor-1 (CSF-1) stimulates DNA synthesis in murine bone marrow-derived macrophages (BMM); however, unlike BMM, murine resident peritoneal macrophages (RPM) undergo a poor proliferative response. It has previously been shown that phosphatidylinositol-4,5-bisphosphate hydrolysis is not associated with CSF-1 action in BMM. In this report we demonstrate that, despite a lack of inositol trisphosphate generation, CSF-1 transiently elevated both [3H]myristoyl- and [3H]arachidonyl-diacylglycerol (DAG) in BMM in a dose-dependent fashion. CSF-1 failed, however, to stimulate an increase in either species of DAG in RPM. Thus, DAG could be a second messenger for the proliferative action of CSF-1 in macrophages. Other mitogenic agents, 12-0-tetradecanoyl phorbol 13-acetate (TPA) and exogenous phospholipase C, also increased BMM levels of [3H]myristoyl- and [3H]arachidonyl-DAG. The nonmitogenic agents, lipopolysaccharide (LPS), tumor necrosis factor-alpha (TNF-alpha) and zymosan, had different effects on the generation of either species of DAG in BMM. LPS failed to elevate either form, TNF-alpha increased only [3H]arachidonyl-DAG, while zymosan stimulated levels of both species of DAG. It therefore appears that increased diacylglycerol generation may be necessary, but perhaps not sufficient, for macrophage proliferation.

Diversity in receptor signalling: cellular individuality and the search for selective drugs

Recent developments in the molecular pharmacology of receptor signalling are reviewed. Receptors and other signalling molecules belong to superfamilies characterized by variations on given themes. A remarkable degree of diversity is generated by unique combinations of molecular entities, and there are also interactions between parallel signal transduction pathways. Molecular biology techniques have revealed new targets for drug development, as well as new methods of drug testing. However, it is not yet clear whether molecular and biochemical specificity always correspond to clinical selectivity.

Tumor necrosis factor-alpha stimulates phosphatidylinositol breakdown by phospholipase C to coordinately increase the levels of diacylglycerol, free arachidonic acid and prostaglandins in an osteoblast (MC3T3-E1) cell line

The effects of (human recombinant) tumor necrosis factor-alpha on phosphatidylinositol breakdown, release of 1,2-diacylglycerols, mobilization of arachidonate from diacylglycerol and prostaglandin synthesis were examined in a model osteoblast cell line (MC3T3-E1). Tumor necrosis factor-alpha (10 nM) caused a specific (30%) decrease in the mass of phosphatidylinositol (and no other phospholipids) within 30 min of exposure. Tumor necrosis factor-alpha doubled the rate of incorporation of [32P]orthophosphoric acid into phosphatidylinositol, indicating that the turnover of inositol phosphate was enhanced, and increased the content of diacylglycerol in parallel with phosphatidylinositol breakdown. The cytokine (10-50 nM; 4 h) also promoted a specific release of 24-34% of the [3H]arachidonate from prelabeled phosphatidylinositol, a release of 80% of the 3H-fatty acid from the diacylglycerol pool, and a 30-fold increase in the synthesis of prostaglandin E2. The tumor necrosis factor-alpha induced liberation of [3H]arachidonate from diacylglycerol, cellular arachidonate release and the synthesis of prostaglandin E2 were each blocked by an inhibitor of diacylglycerol lipase, the compound RHC 80267 (30 microM). Therefore, we conclude that, in the MC3T3-E1 cell line, tumor necrosis factor-alpha activates a phosphatidylinositol-specific phospholipase C (phosphatidylinositol inositolphosphohydrolase; EC 3.1.4.3) to release diacylglycerol, and increases the metabolism of diacylglycerol to liberate arachidonate for prostaglandin synthesis.

Bradykinin activates a phospholipase D that hydrolyzes phosphatidylcholine in PC12 cells

In PC12 pheochromocytoma cells whose phospholipids had been prelabelled with [3H]palmitic acid, bradykinin increased the production of [3H]phosphatidic acid. The increase in [3H]phosphatidic acid occurred within 1-2 min. before the majority of the increase in [3H]diacylglycerol. When the phospholipids were prelabeled with [3H]choline, bradykinin increased the intracellular release of [3H]choline. The production of phosphatidic acid and choline suggests that bradykinin was increasing the activity of phospholipase D. Transphosphatidylation is a unique property of phospholipase D. In cells labeled with [3H]palmitic acid, bradykinin stimulated the transfer of phosphatidyl groups to both ethanol and propanol to form [3H]phosphatidylethanol and [3H]phosphatidylpropanol, respectively. The effect of bradykinin on [3H]phosphatidic acid and [3H]phosphatidylethanol formation was partially dependent on extracellular Ca2+. In cells treated with nerve growth factor, carbachol also increased [3H]phosphatidylethanol formation. To investigate the substrate specificity of phospholipase D, cells were labeled with [14C]stearic acid and [3H]palmitic acid, and then incubated with ethanol in the absence or presence of bradykinin. The 14C/3H ratio of the phosphatidylethanol that accumulated in response to bradykinin was almost identical to the 14C/3H ratio of phosphatidylcholine. The 14C/3H ratio in phosphatidic acid and diacylglycerol was higher than the ratio in phosphatidylcholine. These data provide additional support for the idea that bradykinin activates a phospholipase D that is active against phosphatidylcholine. The hydrolysis of phosphatidylcholine by phospholipase D accounts for only a portion of the phosphatidic acid and diacylglycerol that accumulates in bradykinin-stimulated cells: bradykinin evidently stimulates several pathways of phospholipid metabolism in PC12 cells.

5-hydroxytryptamine-stimulated accumulation of 1,2-diacylglycerol in the rabbit basilar artery: a role for protein kinase C in smooth muscle contraction

1. 5-Hydroxytryptamine (5-HT) produced a concentration-dependent increase in the membrane concentration of 1,2-diacylglycerol (DG) in the rabbit isolated basilar artery, but did not stimulate the hydrolysis of membrane phosphoinositide. 2. The 5-HT-induced accumulation of DG could be blocked with the putative phospholipase C inhibitor 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC; 70 microM), but not with the protein kinase C inhibitor, 1-(5-isoquinolinesulphonyl)-2-methyl piperazine (H7; 50 microM). 3. Direct stimulation of protein kinase C with phorbol 12,13-dibutyrate (PDBu) produced sustained smooth muscle contraction which was fairly rapid in onset and could be reversed by H7 but not by NCDC. The inactive phorbol, 4 alpha phorbol 12,13-dideceonate, did not produce contraction in the basilar artery. 4. 5-HT-induced contractions (1 nM-100 microM) were blocked or greatly reduced in the presence of the protein kinase inhibitor H7 or polymyxin B, and with the phospholipase C inhibitor, NCDC. The concentrations of these inhibitors which abolished contraction to 5-HT, did not alter smooth muscle contraction produced in response to 30 mM K(+)-physiological salt solution (PSS). 5. These data suggest that DG production and the subsequent activation of PKC forms an important component of the cerebrovascular contractile response to 5-HT. As the DG does not appear to arise from membrane phosphatidylinositol, it appears that 5-HT can stimulate the production of this second messenger in cerebral arteries by a mechanism which is different from peripheral arteries.

Influence of lithium on second messenger accumulation in NG108-15 cells

Treatment of NG108-15 cells with bradykinin for 10 min resulted in a reduction of phosphatidylinositol levels with no change in the other inositol lipids. Accompanying this was a small increase in diacylglycerol but no modification in the level of inositol phosphates. Addition of bradykinin to cells incubated for 30 min in buffer containing lithium did not cause a further decrease in the labeling of phosphatidylinositol, but was accompanied by an increase in mass of the three inositol phosphates. The increase was about 150% for inositol monophosphate, 40% for inositol bisphosphate and 80% for inositol trisphosphate. Under these conditions there was a marked increase in the ability of bradykinin to elevate the content of diacylglycerol. Moreover lithium by itself induced an increase in diacylglycerol six times as great as the increase in total inositol phosphates. The excess diacylglycerol may have its origin in phospholipids other than phosphoinositides.

Prolactin-stimulated ornithine decarboxylase induction in rat hepatocytes: coupling to diacylglycerol generation and protein kinase C

The trophic effects of prolactin (PRL) in rat liver have been linked to activation of protein kinase C (PKC). Since alterations in PKC activity imply its activation by 1,2-diacylglycerol (DAG), we tested whether PRL treatment stimulated DAG generation coupled to induction of a growth response in primary hepatocytes. Addition of PRL to hepatocyte cultures significantly increased [3H]-glycerol incorporation into DAG within 5 minutes which was followed by a loss of cytosolic PKC activity by 10 minutes. Prolactin also significantly enhanced radiolabel incorporation into triacylglycerol and phospholipids within 10 minutes and induced ODC activity at 6 hours. Therefore, prolactin-stimulated alterations in PKC activity are preceded by enhanced DAG generation. Moreover, these events appear to be coupled to PRL-stimulated entry of hepatocytes into cell cycle.

Alteration of 1,2-diacylglycerol content in ischemic and reperfused heart

The myocardial 1,2-diacylglycerol (DG) and phospholipid levels during ischemia and reperfusion were studied in open-chest dogs by means of sequential epicardial minibiopsies, followed by quantification based on mass measurement technique. 1,2-DG level increased as early as 5 min after coronary ligation but decreased at 30 min. Also as early as 2 min after postischemic (35 min) reperfusion, 1,2-DG level increased transiently compared to pre-reperfusion level. Prazosin inhibited these changes significantly. A significant change in the incidence of reperfusion-induced ventricular tachycardia (VT) was not obtained in the prazosin-treated group. However, the 1,2-DG level 2 min after reperfusion was significantly higher in the ischemic myocardium developed reperfusion-induced VT than in the undeveloped one. Phospholipid levels remained unchanged during ischemia and reperfusion. These results suggest that alpha 1-adrenergic stimulation occurs early in ischemia and reperfusion and leads to 1,2-DG accumulation, which may be involved in the pathogenesis of ischemic and reperfusion injury.

Identification and characterization of angiotensin II receptor subtypes in rabbit ventricular myocardium

CGP 42 112 A and DuP 753 block [125I]-angiotensin II binding in rabbit ventricular myocardial membranes in a clearly biphasic manner, indicating the existence of high- and low-affinity sites for these highly selective agents. Assays using concentrations of either agent large enough to prevent high-affinity binding show that their respective high-affinity sites are distinct, and each corresponds to the low-affinity site of the other. The two receptor subsets, present in nearly equal proportions, are also distinguishable by their different sensitivities to dithiothreitol. These findings afford strong evidence for the existence of two distinct angiotensin II receptors in rabbit myocardium, corresponding to the A and B subtypes recently described in adrenals.

Purification, cDNA-cloning and expression of human diacylglycerol kinase

Diacylglycerol (DG) kinase attenuates the level of the second messenger DG in signal transduction, and therefore possibly modulates protein kinase C (PKC). DG kinase was purified to homogeneity from human white blood cells, showing an Mr of 86 kDa as determined by SDS-PAGE and gel filtration. Two amino acid sequences of tryptic peptides from DG kinase were determined and degenerate oligonucleotides were prepared and used in the polymerase chain reaction. An amplified DNA fragment was subsequently used to clone the full-length human DG kinase cDNA. This sequence is the human homolog of a porcine DG kinase cDNA sequence reported recently. The sequence contains a double EF-hand structure typical for Ca2+ binding proteins. DG kinase further contains a double cysteine repeat that is present in all PKC isoforms, where it constitutes the phorbol ester (and most likely diacylglycerol) binding site. Therefore we speculate that the double cysteine repeat in DG kinase is involved in DG binding. DG kinase is transcribed as a single mRNA of 3.2 kb, that is highly expressed in T-lymphocytes. The human DG kinase cDNA when transfected in mammalian cells (COS-7) results in a 6-7-fold increase of DG kinase activity.

A method for the quantitative analysis of molecular species of alkylacylglycerol and diacylglycerol

We describe a method for the quantitative analysis of molecular species of diacylglycerol and alkylacylglycerol as their diradylglycerobenzoate derivatives. Synthetic internal standards were used to provide quantitative determinations of the low levels of diacylglycerol and alkylacylglycerol and their individual molecular species in cultured cells. Diradylglycerols were isolated by thin-layer chromatography (TLC), converted to their benzoate derivatives and separated into subclasses by TLC. The molecular species of each subclass were analyzed by reversed-phase high performance liquid chromatography. Thirty-six species of diglyceride-type molecules were identified in Madin-Darby canine kidney cells. These cells were shown to contain 7.88 nmoles of diacylglycerol and 3.97 nmoles of alkylacylglycerol per mumole of phospholipid. Both subclasses contain predominantly monoenoic and saturated species. This technique should be valuable for studies examining the origin and metabolism of these important intracellular mediators.

Vasopressin stimulates phospholipase D activity against phosphatidylcholine in vascular smooth muscle cells

It is now clear that various hormones and agonists can stimulate the production of lipid mediators from non-phosphoinositide phospholipids. We have investigated the production of diacylglycerol from nonphosphoinositide sources, and we demonstrated that vasopressin and other vasoactive agents stimulate hydrolysis of phosphatidylcholine in a variety of cultured vascular smooth muscle cells of rat and human origin. We used vasopressin to characterize this response and found that vasopressin stimulates phospholipase D activity against phosphatidylcholine in A-10 vascular smooth muscle cells. The vasopressin-stimulated phosphatidylcholine hydrolysis is both time- and concentration-dependent. The half-maximal dose of vasopressin required for phosphatidylcholine hydrolysis (ED50 approximately 1 nM) correlates well with vasopressin binding to A-10 cells (Kd approximately 2 nM). The phosphatidylcholine in A-10 cells can be preferentially radiolabeled with [3H]myristic acid; subsequent treatment with vasopressin stimulates a rapid increase in 3H-labeled phosphatidate (approximately 4 X control values at 3 min), and after a short lag, 3H-labeled diacylglycerol rises and reaches maximal levels at 10 min (approximately 2 X control values). Similar temporal elevations of phosphatidate and diacylglycerol occur in A-10 cells labeled with [3H] glycerol. In A-10 cells radiolabeled with [3H] choline, the elevation of cellular phosphatidate and diacylglycerol is concomitant with the release of [3H] choline metabolites (predominantly choline) to the culture medium. The temporal production of phosphatidate and diacylglycerol as well as the release of choline to the culture medium are consistent with vasopressin activating phospholipase D. In addition, vasopressin stimulates a transphosphatidylation reaction that is characteristic of phospholipase D. The transphosphatidylation reaction is detected by the production of phosphatidylethanol that occurs when A-10 cells are incubated with ethanol and stimulated with vasopressin. The phospholipase D is active in the absence of extracellular Ca++ whereas the vasopressin-stimulated mobilization of arachidonic acid is dependent on extracellular Ca++. The data indicate that vasopressin stimulates phospholipase D which hydrolyzes phosphatidylcholine to phosphatidate. The phosphatidate is then metabolized, presumably by a phosphatidate phosphohydrolase, to produce sustained levels of cellular diacylglycerol. These sustained levels of diacylglycerol may activate protein kinase C and thereby function in the "sustained phase" of cellular responses.

Cooperative homotropic interaction of L-noradrenaline with the catalytic site of phenylalanine 4-monooxygenase

Catecholamines (adrenaline, noradrenaline and dopamine) are potent inhibitors of phenylalanine 4-monooxygenase (phenylalanine hydroxylase, EC 1.14.16.1). The amines bind to the enzyme by a direct coordination to the high-spin (S = 5/2) Fe(III) at the active site (charge transfer interaction), as seen by resonance Raman and EPR spectroscopy. Experimental evidence is presented that a group with an apparent pKa value of about 5.1 (20 degrees C) is involved in the interaction between the catecholamine and the enzyme. The high-affinity binding of L-noradrenaline to phenylalanine hydroxylase, as studied by equilibrium microdialysis (anaerobically) and ultrafiltration (aerobically), shows positive cooperativity (h = 1.9); at pH 7.2 and 20 degrees C the rat enzyme binds about 0.5 mol L-noradrenaline/mol subunit with a half-maximal binding (S50) at 0.25 microM L-noradrenaline. No binding to the ferrous form of the enzyme was observed. The affinity decreases with decreasing pH, by phosphorylation and by preincubation of the enzyme with the substrate L-phenylalanine, while it increases after alkylation of the enzyme with the activator N-ethylmaleimide. Preincubation of the enzyme with L-phenylalanine also leads to a complete loss of the cooperativity of L-noradrenaline binding (h = 1.0). The many similarities in binding properties of the inhibitor L-noradrenaline and the activator/substrate L-phenylalanine makes it likely that the cooperative interactions of these effectors are due to their binding to the same site. The high-affinity of catecholamines to phenylalanine hydroxylase is a valuable probe to study the active site of this enzyme and is also relevant for the homologous enzyme tyrosine hydroxylase, which is purified as a stable catecholamine-Fe(III) complex.

Kinetic and molecular species analyses of mitogen-induced increases in diglycerides: evidence for stimulated hydrolysis of phosphoinositides and phosphatidylcholine

A wide variety of agonist-induced events appear to be mediated through an increase in cellular diglyceride levels. With regard to the ability of diglycerides to mediate these events, three important parameters must be considered: a) the kinetics of diglyceride generation, b) the absolute mass levels, and c) their molecular species. While this increase is often due to a stimulated hydrolysis of phosphoinositides, there is increasing evidence that the stimulated hydrolysis of phosphatidylcholine also contributes to agonist-induced increases in diglyceride levels. The kinetics of mass increases in diglyceride levels stimulated in cultured fibroblasts are agonist-dependent. High concentrations of alpha-thrombin stimulate a biphasic increase in diglyceride levels with the first phase peaking at 15 s and the second phase peaking at 5 min. In contrast, stimulation with epidermal growth factor, or platelet-derived growth factor, results in a monophasic increase in cellular diglyceride levels. Furthermore, the molecular species and phospholipid source of the stimulated diglycerides are also agonist-dependent. While the hydrolysis of phosphoinositides is major source of diglycerides initially generated in response to some agonists (15 s with alpha-thrombin at 500 ng/ml), phosphatidylcholine is hydrolyzed as well. Following longer incubations, or at all times following stimulation by epidermal growth factor or platelet-derived growth factor, phosphatidylcholine hydrolysis is the principal source of the stimulated diglycerides.

Low diacylglycerol values in colonic adenomas and colorectal cancer

The biochemical events that make colonic epithelial cells proceed along the adenoma-carcinoma sequence are not well understood. The phosphoinositol signal transduction pathway is involved in the regulation of cell growth and differentiation. To determine its role in colonic neoplasias we performed mass measurements of its second messenger sn-1,2-diacylglycerol in biopsy specimens from normal mucosa and neoplasias of the colon. Normal colonic mucosa was also investigated in patients without colonic abnormalities (n = 10). Compared with pooled diacylglycerol values from five colonic sites (100%), values in patients with a normal colon were highest in the ascending colon (120 (5)%, p less than 0.05) and lowest in the rectum (81 (5)%, p less than 0.01). Absolute diacylglycerol values in patients with normal colons (2.62 (0.16) nmol/mg protein) were not significantly different from those found in the normal mucosa of patients with colorectal neoplasias (2.45 (0.17) nmol/mg protein). Both colonic adenomas (n = 15) and colorectal carcinomas (n = 14) showed significantly decreased diacylglycerol values compared with the adjacent normal mucosa of each patient (72 (4)%, p less than 0.001, and 71 (4)%, p less than 0.001 respectively). The appreciable decrease in mass diacylglycerol values clearly distinguishes adenomas and carcinomas of the colon from the surrounding normal mucosa. This finding suggests that profound metabolic changes of the phosphoinositol signal transduction pathway occur early in the adenoma-carcinoma sequence and may be important in colonic carcinogenesis.

Ca2(+)-dependence of arachidonic acid redistribution among phospholipids of cultured mouse keratinocytes

Mouse keratinocytes cultured in a medium containing less than 0.1 mM Ca2+ (low Ca2+) incorporated [1-14C]arachidonic acid (AA) into phospholipids by kinetics including; (i) a rapid labelling of phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer) and both acid-stable and alkenylacyl forms of phosphatidylcholine (PtdCho); and (ii) a slow but long-lasting radiolabel incorporation into both acid-stable and alkenylacyl forms of phosphatidylethanolamine (PtdEtn), partly associated with a net radioactivity loss from acid stable-PtdCho. Under low Ca2+ conditions no radioactivity transfer apparently occurred between PtdIns and other phospholipid classes. When cells were prelabelled for 24 h with [1-14C]AA and reincubated in label-free medium containing 1.2 mM Ca2+ (normal Ca2+), an early and extensive loss of radioactivity from PtdIns was observed, reasonably in connection with Ca2+ stimulation of phosphoinositide turnover. Cell shift to normal Ca2+ did not result in an increased synthesis of labelled eicosanoids, but was consistent with an increase of radioactivity incorporation into diacylglycerol (DAG) and with a complex pattern of [1-14C]AA redistribution, eventually leading to a marked radioactivity incorporation into acid stable-PtdEtn (but not into alkenylacyl-PtdEtn) and to a labelling decrease of acid stable-PtdCho. The possible mechanisms driving AA recycling after cell shift to normal Ca2+ are discussed.

sn-1,2-diacylglycerol levels increase in progesterone-stimulated Xenopus laevis oocytes

Full-grown Xenopus laevis oocytes resume meiosis from prophase arrest in response to progesterone stimulation. Recent studies have shown that the tumor promoter, 12-O-tetradecanoylphorbol 13-acetate (TPA), a very potent activator of protein kinase C, can also induce the resumption of meiosis in amphibian oocytes. We have investigated the possibility that sn-1,2-diacylglycerol (DAG), the intracellular activator of protein kinase C, may be involved in the pathway normally used by progesterone. We have found that full-grown oocytes arrested in meiotic prophase contain 48 +/- 5 pmoles DAG/oocyte. This level increases within the first 5 minutes following the addition of progesterone and reaches a maximum of 75 +/- 10 pmoles following 60 minutes of steroid stimulation. Progesterone induces the up-regulation of a Na+/H+ antiport in the plasma membrane of the oocyte causing an increase in pHi following 60 minutes of steroid stimulation. We have found that the addition of 200 microM DAG (1,2-dioctanoylglycerol, diC8) to the culture medium can cause a partial up-regulation of this Na+/H+ pump in the absence of hormonal stimulation. These results suggest that DAG and protein kinase C may be involved in regulating certain aspects of meiotic maturation in progesterone-stimulated Xenopus oocytes.

Discrimination of two angiotensin II receptor subtypes with a selective agonist analogue of angiotensin II, p-aminophenylalanine6 angiotensin II

Angiotensin II receptor binding sites in rat liver and PC12 cells differ in their affinities for a nonpeptidic antagonist, DuP 753, and p-aminophenylalanine6 angiotensin II. In liver, which primarily contains the sulfhydryl reducing agent-inhibited type of angiotensin II receptor, which we refer to as the AII alpha subtype, DuP 753 displays an IC50 of 55 nM, while p-aminophenylalanine6 angiotensin II displays an IC50 of 8-9 microM. In PC12 cells, which primarily contain the angiotensin II receptor type whose binding affinity is enhanced by sulfhydryl reducing agents (AII beta), DuP 753 displays an IC50 in excess of 100 microM, while p-aminophenylalanine6 angiotensin II displays an IC50 of 12 nM. p-Aminophenylalanine6 angiotensin II binding affinity in liver is decreased in the presence of guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) suggesting that this analogue is an agonist.

Phospholipase C-mediated hydrolysis of phosphatidylcholine is an important step in PDGF-stimulated DNA synthesis

Recent evidence suggests the involvement of phosphatidylcholine (PC) hydrolysis both in the control of normal cell growth and in transformation. We show here that the simple exogenous addition of Bacillus cereus PC-hydrolyzing phospholipase C (PC-PLC) is sufficient to elicit a potent mitogenic response in Swiss 3T3 fibroblasts by a mechanism that is independent of protein kinase C. Our results on the additivity and synergism between B. cereus PC-PLC, PDGF, and insulin in the mitogenic response indicate that this novel phospholipid degradative pathway may be important in the mitogenic signaling cascade activated by PDGF.

Activation of platelets induced by mAb P256 specific for glycoprotein IIb-IIIa. Possible evidence for a role for IIb-IIIa in membrane signal transduction

Monoclonal antibody P256, which is specific for glycoprotein IIb-IIIa complex, was found to induce aggregation of normal platelets in plasma. The mechanism of platelet activation induced by this monoclonal antibody was thoroughly studied. The divalent binding to the IIb-IIIa molecule was necessary for triggering aggregation since Fab' fragments did not induce aggregation as did IgG and F(ab')2 fragments; however, F(ab')2 did not induce the release as did the whole IgG. P256-induced aggregation was accompanied by release of all three granule constituents, namely dense granules, alpha-granules and lysosomes, with parallel kinetics showing half-maximum release 50 s after addition of P256. Thromboxane synthesis was initiated at the same time. Using 32P-prelabeled platelets, no variation in level of [32P]phosphatidylinositol 4,5-bisphosphate could be detected in the first minute after P256 addition, indicating no activation of the calcium-independent phospholipase C specific for polyphosphoinositol phospholipid. P256 induced a calcium mobilization as measured by Indo-1 fluorescence of about the third of that measured in the presence of a thrombin concentration giving the same intensity of aggregation. P256 induced phosphorylation of the myosin light chain p20 and of the main substrate of protein kinase C, p43. Addition of aspirin inhibited almost totally calcium mobilization and partially aggregation, release and protein phosphorylations. By contrast, in the absence of external calcium, although no aggregation could occur, the release reaction was only partially reduced. In this activation, the glycoprotein IIb-IIIa complex thus appears to play a role in modulating platelet response, not only via calcium fluxes but also in activating protein kinase C responsible for p43 phosphorylation.